Mobile-use fuel cells are now under development, and an air pump that is suitable for supplying air to fuel cells has not yet been developed. For such air pump, supplied air should not include impurities such as oil, that is, the pump should be an oilless mechanism, the amount of supplied air should be 5 to 10 L/min, but pressure should be about ΔP=5 kPa to compensate pressure loss in the air passage of a fuel cell and blow the air, the diameter should be substantially 30 mm or less depending on the usage in a mobile, noise should be of low level, and the lifetime of several tens of thousands of hours should be achieved. Achieving the lifetime is particularly important among the foregoing characteristics and the component determining the lifetime is a bearing. A method improving the lifetime of a bearing known in the art is to form a lead groove and an oil storage (space) to prevent the oil from leaking outside (e.g. JP-A-7-312843).
FIG. 8 shows a fan motor disclosed in the above Patent Document of JP-A-7-312843. As for the configuration of the fan motor, metal sleeves 3 are arranged as bearings in a bearing stand 2 that is press-fitted in a casing 1, and a shaft 4 is inserted in the sleeves. A spiral groove 4′ is formed along the surface of the shaft 4. As the shaft 4 rotates, oil leaking out between the metal sleeves 3 and the shaft 4 is guided into an oil storage 8 through the spiral groove 4′, thereby preventing the oil from flowing outside.